Abstract
Taurine bromamine (Tau-NHBr) is produced by the reaction between hypobromous acid (HOBr) and the amino acid taurine. There are increasing number of applications of Tau-NHBr as an anti-inflammatory and microbicidal drug for topical usage. Here, we performed a comprehensive study of the chemical reactivity of Tau-NHBr with endogenous and non-endogenous compounds. Tau-NHBr reactivity was compared with HOBr, hypochlorous acid (HOCl) and taurine chloramine (Tau-NHCl). The second-order rate constants (k2) for the reactions between Tau-NHBr and tryptophan (7.7 × 102 M−1s−1), melatonin (7.3 × 103 M−1s−1), serotonin (2.9 × 103 M−1s−1), dansylglycine (9.5 × 101 M−1s−1), tetramethylbenzidine (6.4 × 102 M−1s−1) and H2O2 (3.9 × M−1s−1) were obtained. Tau-NHBr demonstrated the following selectivity regarding its reactivity with free amino acids: tryptophan > cysteine ~ methionine > tyrosine. The reactivity of Tau-NHBr was strongly affected by the pH of the medium (for instance with dansylglycine: pH 5.0, 1.1 × 104 M−1s−1, pH 7.0, 9.5 × 10 M−1s−1 and pH 9.0, 1.7 × 10 M−1s−1), a property that is related to the formation of the dibromamine form at acidic pH (Tau-NBr2). The formation of singlet oxygen was observed in the reaction between Tau-NHBr and H2O2. Tau-NHBr was also able to react with linoleic acid, but with low efficiency compared with HOBr and HOCl. Compared with HOBr, Tau-NHBr was not able to react with nucleosides. In conclusion, the following reactivity sequence was established: HOBr > HOCl > Tau-NHBr > Tau-NHCl. These findings can be very helpful for researchers interested in biological applications of taurine haloamines.
Highlights
Hypochlorous acid (HOCl) and hypobromous acid (HOBr) are microbicidal agents produced when the white blood cells, neutrophils and eosinophils, respectively, are challenged by stimuli like bacteria and fungi [1]
It exists in equilibrium with its dibromamine form (Tau-NBr2 ) and, as has been demonstrated by Thomas et al, pure Tau-NHBr is only obtained using a large excess of taurine
2016, 6, 23 chloramine and bromamine are in equilibrium with their dichloramine and dibromamine forms [32,33]
Summary
Hypochlorous acid (HOCl) and hypobromous acid (HOBr) are microbicidal agents produced when the white blood cells, neutrophils and eosinophils, respectively, are challenged by stimuli like bacteria and fungi [1]. While MPO-I (1.16 V) is able to oxidize efficiently both Cland Br [11]; EPO-I (1.09 V) shows a large preference for Br [12] This chemical feature, the higher plasma level of Cl (~100 mM) compared with Br (~100 μM), and the higher oxidant capacity of HOCl (1.28 V) compared with HOBr (1.13 V) [13], could suggest that the biological effect of HOBr would be irrelevant compared with HOCl. This chemical feature, the higher plasma level of Cl (~100 mM) compared with Br (~100 μM), and the higher oxidant capacity of HOCl (1.28 V) compared with HOBr (1.13 V) [13], could suggest that the biological effect of HOBr would be irrelevant compared with HOCl This does not seem to be the case, since there is significant evidence that HOBr is more reactive with several biomolecules compared to HOCl [14,15]. Yazdanbakhsh et al provided the first application of Tau-NHBr and proposed its endogenous formation [17] These authors found that stimulated eosinophils could be a source of Tau-NHBr, since these cells may produce HOBr, and taurine is abundantly present in leukocytes. We believe that this chemical data will be helpful for those interested in the application of this interesting compound
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